Mobile IP protocol is an Internet Engineering Task Force (IETF) standard communications protocol. Mobile IP protocol allows mobile clients to move from one network to another network while maintaining the same Internet Protocol (IP) address. Usually, each mobile client is identified by its home address regardless of its current location. While away from its home network, the mobile client is associated with a care-of address which identifies its current location and its home address is associated with the local endpoint of a tunnel to its home agent. Thus, Mobile IP protocol specifies how a mobile client registers with its home agent and how the home agent routes datagrams to the mobile client through the tunnel.
Conventionally, mobility in the network layer has been achieved with Internet Protocol in Internet Protocol (IPIP) tunnels, which carry only the Internet Protocol (IP) payload between a home agent and a foreign agent in a wireless network. Original physical layer headers are not carried across the tunnel. Thus, the only means of identifying the client will be based on the IP address.
Also, the traffic has to be routed once the packets reach the home agent. Such routing by the home agent introduces at least the following issues: First, it requires routing to be enabled by the home agent, where the mobile client was initially associated with, on its home virtual local area network (VLAN). Second, when packets are routed using the best route on the home agent, the routed traffic could leave the home agent on a different VLAN than the home VLAN of the home agent. Third, when network layer broadcast or multicast traffic from a roaming mobile client is received by the home agent, the VLAN used to forward the traffic has to be decided implicitly based on reverse route lookup by the source IP address or client IP address of the packet.
On the home agent, if the traffic is routed on a different VLAN than the home VLAN, it may potentially allow guest traffic on a different VLAN that is not the home VLAN of the home agent. This can cause traffic to be black-holed due to the following reasons: First, the receiving router can have anti-spoofing access control lists (ACLs) that are configured to determine which packets are to be dropped based on whether the source IP corresponds to the incoming VLAN subnet.
Second, the receiving router has virtual routing and forwarding (VRF) configured, and thus could prevent guest traffic from being forwarded outside a specific VRF path. Home VLAN has been configured to perform source NAT on the traffic forwarded out of the vlan. If the traffic is not forwarded from the Home Vlan, the home agent may fail to source NAT the packet.
Moreover, conventional mobility implementations trap DHCP messages from the mobile client and play them back on the home VLAN on behalf of the mobile client. This also ensures that DHCP transaction does not get transmitted on a foreign VLAN, because transmitting DHCP transaction on a foreign VLAN would result in the mobile client changing its IP address. When implemented in control plane, the conventional mobility implementations do not scale when a high density of mobile clients roam during the same time period. When a mobile client roams away from its home agent, the DHCP packets are relayed from the foreign agent to the home agent. Therefore, the relay process introduces undesirable latency, since the same packets have to travel multiple hops before making to their final destination.